1. Field of the Invention
The present invention relates to an inspection system, and in particular to an extreme ultraviolet (EUV) high throughput inspection system.
2. Related Art
Current patterned mask inspection is typically accomplished using transmissive optics and coherent sources. Specifically, the large magnifications provided by transmissive microscopes of moderate NA (numerical aperture) systems coupled to deep UV (ultraviolet radiation, e.g. 257 nm, 193 nm, etc.) coherent illumination sources has been sufficient to provide the sensitivity required for mask defect detection for microelectronics fabricated using excimer laser-based steppers. Moreover, the combined high magnification of the transmissive microscopes and the high brightness of the illumination sources have provided sufficient image throughput to ensure cost effective mask inspection systems for deep UV mask inspection.
Extreme ultraviolet (EUV) mask inspection systems are known. Unfortunately, the optics of these systems are relatively low resolution and have an inadequate field of view at the image plane for the cost-effective inspection of EUV patterned masks. Therefore, a need arises for an inspection system with a field of view at the image plane consistent with high resolution and cost-effective EUV patterned mask, mask blank, and wafer inspection.
EUV sources of high average power have been previously described in lithography applications with etendue (i.e. light spread as defined by area and angle) requirements of 1-3.3 mm2-sr and average power of 210 W at 13.4 mm at the intermediate focus. These EUV sources have typically included discharge-driven or laser-driven plasmas. Unfortunately, none of these EUV sources can efficiently generate radiation within the etendue required for EUV mask inspection applications because the conversion efficiency of lasers or discharges to EUV photons is highly inefficient, e.g. in the range of only 1-3% conversion. Therefore, a further need arises for an EUV source that can minimize power consumption. In particular, a need arises for a laser driver of average power and repetition rate that also minimizes debris effects on the collector optics.
As noted above, conventional inspection systems use coherent sources of wavelengths, e.g. ≧193 nm. However, state of the art masks are being produced using EUV radiation at ≦13 nm. With two such disparate wavelengths, the inspection of masks with conventional inspection systems can be problematic, particularly with regard to the interpretation and rendering of optical proximity effects. In other words, the sensitivity of the 193 nm based mask inspection systems can be inadequate for EUV masks with features on the order of 13 nm. Therefore, a further need arises for an illumination source that can provide adequate sensitivity for the inspection of EUV masks.